The present invention relates to compositions comprising blends of polypropylene and thermoplastic block copolymers.
Many polymer blends are known and used in different industries. For example, polymer blends are important in the development of pressure-sensitive adhesives that are used to make tapes and transdermal patches. The characteristics of a pressure sensitive adhesive depends on the amount and the types of polymers used to make the adhesive. Preferred pressure sensitive adhesives include rubber-based adhesives, which consist of blends of natural or synthetic rubber and tackifier resins. Typically the use of thermoplastic block copolymers in adhesive compositions provide strength and shear resistance in the adhesive. Other examples that include polypropylene and thermoplastic block copolymer blends are used to produce compositions amiable to molding processes. The resultant thermoplastic blends having high impact resistance, good oxidative stability, and are used in a variety of applications.
The thermoplastic block copolymers used in polymer blends are a unique class of elastomers. Some block copolymers have hard segments of polystyrene combined with soft segments, such as the commercially available KRATON thermoplastic polymers, sold by the Shell Chemical Company, Houston, Tex. These thermoplastic block copolymers can be blended with many plastics, such as styrenics, polyolefins and engineering thermoplastics to improve their performance, particularly their impact properties. These thermoplastic block copolymers toughen such plastics both at room and low temperatures and can also make dissimilar plastics compatible. And with the right grade of thermoplastic block copolymers, such toughening or compatibilization can be achieved efficiently, while retaining high heat resistance, good processability, good weatherability, and in certain cases, clarity.
Mixing thermoplastic block copolymers with plastics typically produce compositions having measured physical characteristics between the measured physical characteristics of the plastic or the thermoplastic block copolymer used to make the compositions. Specifically, compositions produced by mixing a thermoplastic block copolymer and an isotactic polypropylene having about 100 percent isotactic linkages will usually have toughness and abrasion resistance measurements between the toughness and abrasion resistance measurements of the specified thermoplastic block copolymer and the specified polypropylene used to make the composition.
The present invention embodies blends including polypropylenes and thermoplastic block copolymers, wherein the blend has specific physical characteristic more desirable than the physical characteristic of the polypropylene and the thermoplastic block copolymer used to make the blend. One embodiment of the invention is a composition comprising a polypropylene including less than 90 percent isotactic linkages, and a thermoplastic block copolymer comprising hard segments of polystyrene combined with soft segments comprising carbon chains including 2 to 8 carbon atoms. The breaking stress measurement of the composition is greater than the breaking stress measurement of the specified polypropylene and the specified thermoplastic block copolymer used to make the composition.
Another embodiment of the invention is a composition comprising a polypropylene comprising about 50 percent to 70 percent isotactic linkages, and a thermoplastic block copolymer comprising hard segments of polystyrene combined with soft segments including ethylene and butylene. This composition contains some physical characteristics that are more desirable than the physical characteristics of the polypropylene and the thermoplastic block copolymer used to make the composition. Such physical characteristics include tensile breaking stress, abrasion resistance, and percent elongation at break.
Another embodiment of the invention is a composition comprising a polypropylene including less than 90 percent isotactic linkages, and a thermoplastic block copolymer comprising hard segments of polystyrene combined with soft segments comprising carbon chains including 2 to 8 carbon atoms. The abrasion resistance measurement (as measured in grams abraded) of the composition is less than the abrasion resistance measurement (as measured in grams abraded) of the specified polypropylene and the specified thermoplastic block copolymer used to make the composition.
Embodiments of the invention generally are compositions that include a polypropylene and a thermoplastic block copolymer.
Polypropylene
Suitable polypropylenes for making a composition of this invention have less than 90 percent isotactic linkages. A blend of these polypropylenes and thermoplastic block copolymers provide a composition having specific physical characteristics more desirable than the polypropylene or thermoplastic block copolymer used to make the composition. Isotactic linkages refers to one of the three arrangements that are possible during the polymerization of propylene in which all methyl groups are on one side of an extended chain. The procedure for measuring the percent isotactic linkages of polypropylene is described in the Example Section of this patent application labeled Procedure II For Measuring The Tacticity Of Polypropylene. Examples of polypropylenes with less than 90 percent isotactic linkages include the products sold under the tradename REXFLEX FPO W101, REXFLEX FPO W108, and REXFLEX FPO W104 by the Rexene Products Company of Dallas, Tex. It is even more desirable that the polypropylenes used in the invention have about 50 percent to 70 percent isotactic linkages.
Thermoplastic Block Copolymers
The composition of this invention includes polypropylene having less than 90 percent isotactic linkages and thermoplastic block copolymers. The thermoplastic block copolymers used to make a layer of cover material are selected based on compatibility with the polypropylene and their ability to provide a composition having specific physical characteristics more desirable than the polypropylene or thermoplastic block copolymer used to make the composition. In general terms, block copolymers are macromolecules comprising chemically dissimilar, terminally connected segments. Their sequential arrangement can vary from Axe2x80x94B structures, containing two segments only, to Axe2x80x94Bxe2x80x94A block copolymers with three segments, to multiblock xe2x80x94(Axe2x80x94B)nxe2x80x94 systems possessing many segments. A unique development resulting from block copolymer technology is the concept of thermoplastic elastomeric behavior. Block copolymer systems of this type are characterized by rubbery behavior in the absence of chemical cross-linking. This feature permits the fabrication of these materials by means of conventional thermoplastic processing techniques. The key to this unique behavior is the ability to achieve a network structure by physical rather than chemical means. This, in turn results from finely controlled morphology in Axe2x80x94Bxe2x80x94A or xe2x80x94(Axe2x80x94B)nxe2x80x94 systems containing both flexible and rigid segments.
The simplest arrangement, or architecture, of a block copolymer is the diblock structure, commonly referred to as an Axe2x80x94B block copolymer, which is composed of one segment of xe2x80x9cAxe2x80x9d repeat units and one segment of xe2x80x9cBxe2x80x9d repeat units. The second form is the triblock, or Axe2x80x94Bxe2x80x94A structure, containing a single segment of B repeat units located between two segments of xe2x80x9cAxe2x80x9d repeat units. The third basic type is the xe2x80x94(Axe2x80x94B)nxe2x80x94 multiblock copolymer, are composed of many alternating xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d blocks. Another, but less common, variation is the radial block copolymer. This structure takes the form of a star-shaped macromolecule in which three or more diblock sequences radiate from a central hub. Suitable thermoplastic block copolymers may have one of the above architectures. It is preferred that the thermoplastic block copolymer in this invention is the triblock, or Axe2x80x94Bxe2x80x94A structure.
From a mechanical property point of view, block copolymers may be conveniently divided, on the basis of room temperature modulus, into two classes-rigid and elastomeric. Rigid materials may be comprised of either two hard segments or of one hard segment together with a minor fraction of a soft segment. A hard segment is defied as one that has a Tg and/or Tm above room temperature, while a soft segment has a Tg (and possibly) a Tm) below room temperature. Elastomeric block copolymers normally contain a soft segment together with a minor proportion of a hard segment. Additional characteristics of block copolymers are described in Block Copolymers Overview and Critical Surveyxe2x80x9d, Allen Noshay, James E, McGrath, Academic Press, 1977, incorporated herein by reference.
It is preferred that thermoplastic block copolymers used in this invention be elastomeric and include hard segments of polystyrene monomer units and soft segments comprising carbon chains including 2 to 8 carbon atoms. Suitable commercially available thermoplastic block copolymers include (SBS) styrene-butadiene-styrene block copolymer, (SIS) styrene-isoprene-styrene block copolymer, (SEBS) styrene-ethylene-butylene-styrene block copolymer, (SEP) styrene-ethylene-propylene block copolymer, (SB)n styrene-butadiene or (SI)n styrene-isoprene multi-arm (branched) copolymer, (EP)n ethylene-propylene multi-arm (branched) polymer. It is preferred that the soft segments of the block copolymer include butadiene, isoprene, ethylene, propylene, butylene, and combinations thereof. It is most preferred that the thermoplastic block copolymer have soft segments comprising ethylene and butylene. It is preferred that the polystyrene segments comprise less than about 30 percent by weight of the block copolymer. It is most preferred that the polystyrene segments comprise about 13 percent by weight of the block copolymer.
Polymer Blends Comprising Polyolefins and Thermoplastic Block Copolymers
Polymer blends of this invention include a polypropylene having less than 90 percent isotactic linkages and a thermoplastic block copolymer. The blends vary in weight percentages of the polypropylene and thermoplastic block copolymer. Preferably, a polymer blend will have about 80 percent by weight of a polypropylene having less than 90 percent isotactic linkages and about 20 percent weight of a thermoplastic block copolymer to about 20 percent by weight of a polypropylene having less than 90 percent isotactic linkages and 80 percent by weight of a thermoplastic block copolymer. Preferably, a polymer blend will have about 60 percent by weight of a polypropylene with less than 90 percent isotactic linkages and about 40 percent by weight of a thermoplastic block copolymer to about 40 percent by weight of a polypropylene with less than 90 percent isotactic linkages and 60 percent by weight of a thermoplastic block copolymer. All polymer blends may also include additives such as fillers, fibers, antistatic agents, lubricants, wetting agents, foaming agents, surfactants, pigments, dyes, coupling agents, plasticizers, suspending agents and the like.
As mentioned, polymer blends including polypropylene having less than 90 percent isotactic linkages and thermoplastic block copolymers provide compositions typically having physical measurements more desirable than the physical measurements of the specified polypropylene or specified thermoplastic block copolymers used to make the blend. These physical characteristics include the tensile breaking stress, percent elongation at break, and abrasion resistance (measured in grams abraded) as illustrated in the Example Section of this patent application. In addition, a specific blend containing a polypropylene having less than 90 percent isotactic linkages typically has a static coefficient of friction enhanced over a similar blend using polypropylene having greater than 90 percent isotactic linkages as illustrated in the Example Section. It is preferred that all blends of this invention have about 10 percent to 40 percent elongation at yield. Preferably, the percent elongation at yield will be between about 13 percent to 40 percent.
The polymer blends have numerous applications and may be used to form mats, mousepads, pull roll levers, anti-slip surfaces, high friction films, abrasion resistant films, conveyor belts, and protective film coatings. For particular applications, the thermoplastic block copolymer selected may have a Shore A hardness of greater than about 30.
Interestingly, some of the compositions embodied by the invention may have abrasion resistance measurements but not tensile breaking stress measurements more desirable than the measurement of the polypropylene having less than 90 percent isotactic linkages and the thermoplastic block copolymer used to make the compositions. An example of such a composition is illustrated in Table 3 where the composition comprises 20 percent Polymer C and 80 percent Polymer A. There are also examples of polymer compositions having tensile breaking stress measurement more desirable than the polypropylene and the thermoplastic block copolymer used to make the blend, but the blend does not have an abrasion resistant measurement more desirable than the abrasion resistance measurement of the polypropylene and the thermoplastic block copolymer used to make the blend. An example of such a composition is illustrated in Table 3 where the composition comprises 40 percent Polymer C and 60 percent Polymer A. A low grams abraded value is desirable. Even though one physical measurement of the blend may be more desirable than the physical measurement of each polymer making up the blend, not all the physical characteristics of the blend measured may behave that way.